A fiber-reinforced plastic made of reinforcing fibers and a matrix resin, also referred to as “FRP,” in particular, a carbon fiber-reinforced plastic made of carbon fibers and a matrix resin, also referred to as “CFRP,” is developed broadly in various uses, because of its excellent mechanical properties, property light in weight, corrosion resistance and the like. As a method of manufacturing an FRP, although there is a method using a so-called “prepreg” which is impregnated with a resin in advance, in a case where it is required to broaden the range of the moldable shape of FRP and to shorten the time required for molding, for example, in a case where a mass production is required such as in parts for vehicles or parts for electronic equipment, a method is frequently employed wherein a reinforcing fiber base material (dry-type reinforcing fiber base material) substantially containing no resin is formed in a predetermined shape, and by impregnating a matrix resin thereinto, a desired FRP is molded.
In such a molding of FRP, in a case where a thermosetting resin is used as the matrix resin, because the viscosity of the thermosetting resin before being cured is low, the resin can be cured after it is impregnated into a reinforcing fiber base material in a mold. In this method, there is an advantage that an expensive autoclave is not required and the time for molding may be relatively short.
On the other hand, to mold an FRP having a more complicated shape, in particular, to conform it also to a mass-production article, it is preferred to use a thermoplastic resin better in moldability as the matrix resin. However, because a thermoplastic resin is generally high in viscosity at the time of being melted as compared to a thermosetting resin, it is frequently difficult to impregnate a melted resin into a reinforcing fiber base material in a short period of time, and it has been difficult to manufacture an FRP with a high productivity by a molding method wherein a resin is impregnated into a reinforcing fiber base material in a mold.
To use a thermoplastic resin as a matrix resin and impregnate it into a reinforcing fiber base material at a relatively good condition, a reinforcing fiber base material is formed in a non-woven fabric-like form by using discontinuous reinforcing fibers, thereby making the porosity of the base material high and facilitating impregnation of the resin having a high viscosity. However, even in such a reinforcing fiber base material having a high porosity, it is not easy to impregnate a thermoplastic resin thereinto, and only a method has been known wherein a matrix resin is once formed in a sheet-like shape such as a non-woven fabric and it is pressed while being heated at a condition of being stacked with a reinforcing fiber base material (for example, JP-A-2010-037358 and JP-A-2010-235779). Even if such a reinforcing fiber base material having a high porosity is placed in a mold for injection molding and a thermoplastic resin is injected, only a CFRP having a very low fiber content or having many pores (voids) has been able to be obtained.
Accordingly, it could be helpful to provide a method of manufacturing a fiber-reinforced plastic which can mold a fiber-reinforced plastic having high mechanical properties easily and inexpensively even if it has a complicated shape.